Adsorption of urea onto granular activated alumina: A comparative study with granular activated carbon

Ultrahigh and kinetic-favorable adsorption for recycling urea using corncob-derived porous biochar

Insufficient attention has been given to the recycling of excess urea despite its potential detrimental effects on soil nutrient equilibrium, geological structure, and crop health. In this study, corncob-derived porous biochar (CPB), which is rich in surface functional groups, was prepared from biomass corncob in two steps as an adsorbent to remove urea from wastewater. Compared with the typical carbonization and activation processes, this process resulted in a higher yield of CPB and an ultrahigh adsorption capacity for urea. Response surface analysis was utilized to determine the optimal carbonization conditions, which were found to be 500 °C for 6 h with a heating rate of 15 °C/min. The exceptional adsorption capability of CPB can be ascribed to its porous structure and significant presence of oxygen-containing functional groups, which facilitate a synergistic interaction of physisorption and chemisorption. This adsorption phenomenon aligns with the Harkins-Jura isotherm model and adheres to pseudo-second order kinetics. CPB demonstrates potential as an adsorbent for the elimination of urea from wastewater in an economical and effective fashion.

Superfast adsorption of small and uncharged urea from water using post-sonicated iron-based metal-organic framework

Urea is widely used in fertilizer production for agricultural purposes which risks runoff into soil and water sources. An excess of urea can result in algal or toxic blooms which can poison wildlife or even humans by accumulation in food sources. The removal of urea from water is challenging due to the small size (0.254 nm) and uncharged surface of urea. Intensive research has been conducted on a variety of methods to remove environmental concentrations of urea using adsorbents, but most of them lack effective removal, require long (>2 h) process time, and lack re-generability. Metal-organic frameworks (MOFs) are the new generation of adsorbents with excellent structural and functional group tunability. In this study, we synthesized MIL-100 (Fe), an iron-based MOF, as an efficient adsorbent for the removal of uncharged urea from water. The urea adsorption capacity of MIL-100 (Fe) was tested under varying experimental conditions such as pH (2-10), temperature (25-65 °C), MOF concentration (25-400 ppm), and urea concentration (25-1000 ppm). The results showed the superfast adsorption (more than 85% removal within 2 min) of neutrally charged urea molecules on MIL-100 (Fe). The MOF was able to reach a maximum adsorption efficiency of around 85% with a maximum uptake capacity of 3321 mg/g. The MIL-100 (Fe) showed acceptable re-generability by retaining up to 90% removal efficiency after four regeneration cycles. The urea adsorption followed pseudo 2nd-order adsorption kinetics and dipole-dipole interactions and π-NH bonding were the primary adsorption mechanism.

Elucidation of the synergistic effects of 3d metal (M = Cu, Co, and Ni) dopants and terminations (T = -O- and -OH) of Ti3C2Tx MXenes for urea adsorption ability via DFT calculations and experiments

Dialysis is an artificial process to remove excess urea toxins from the body through adsorption or conversion. Urea adsorption by emergent 2D materials such as MXenes is one probable approach. Based on density functional theory (DFT) studies, the surface of Ti3C2Tx (T = -O- and -OH) MXenes is not optimum for urea adsorption. Therefore, functionalization with 3d metal dopants (Cu, Co, and Ni) is proposed to improve their urea adsorption ability. DFT calculations indicate that oxygen-terminated Ti3C2O2 has a much better urea adsorption ability when doped with Cu, Co, and Ni, with adsorption energy (Eads) values of -2.11 eV, -1.90 eV and -1.72 eV, respectively. These adsorption energies are much more favourable than that of the undoped one (Eads = -0.52 eV). To verify the calculation results, MILD Ti3C2Tx, or MXenes synthesized via the safer and easier minimally intensive layer delamination (MILD) method, were utilized to simulate Ti3C2O2 since they have -O- termination as the dominant species. Experimentally, the adsorption studies found that low concentration of Cu, Co, and Ni on MILD Ti3C2Tx showed a urea removal efficiency of 21.9%, 6.0% and 0.2%, respectively, much better than 0% removal efficiency of unfunctionalized Ti3C2Tx. Therefore, both DFT calculations and experiments showed that various metal functionalized MXenes have a similar trend for urea adsorption, highlighting the feasibility of using the computational approach to predict urea adsorption and further opening a new promising direction for the urea adsorption. Finally, this study is also the first to examine synergistic effects of metal dopants and surface terminations on MXenes for urea adsorption.

One-Step Hydrothermal Preparation of a Corncob-Derived Porous Adsorbent with High Adsorption Capacity for Urea in Wastewater: Sorption Experiments and Kinetics Study

With rapid industrial development, the massive generation of nitrogenous wastewater poses a serious threat to both human beings and the ecosystem. Bio-based adsorbents are considered promising adsorption materials for many applications. However, their complex preparation procedures, large energy consumption, and difficulty of microstructure control hinder their practical applications. In this study, a new corncob-derived porous adsorbent (CPA) with excellent urea adsorption capacity in wastewater was prepared by the one-step hydrothermal process. The effects of the hydrothermal process conditions on the urea adsorption capacity of the CPA were evaluated and optimized using the response surface methodology, and a kinetic analysis of the CPA was also carried out. Our findings showed that the adsorption process of urea by the adsorbent followed the Langmuir isotherm and pseudo-second-order kinetic models. The high adsorption capacity for urea was attributed to the abundant porous structure and the hydrogen bonds formed between the adsorbent and the amine group in urea, which made it more conducive to the adsorption of urea. Therefore, we believe that CPA could be a promising adsorbent for urea removal in wastewater.

Improving the adsorption performance of urea by using polyhydroxy groups to modify two-dimensional Ti3C2Tx

Wearable artificial kidney can provide continuous dynamic dialysis for uremia patients. For the sake of practical application, the critical step is to find an adsorbent that can effectively remove urea and have excellent biological compatibility. The layered Ti3C2Tx (DL-Ti3C2Tx) with high specific surface area and good dispersion was prepared by a two-step etching method. From the first principles calculation, urea can be adsorbed by different groups (-F, -O, -OH) on the surface of Ti3C2Tx, among which -OH has the greatest binding energy to urea. Therefore, DL-Ti3C2Tx was modified with different alkali solutions (KOH, NaOH, LiOH) to introduce -OH on the surface, which can increase the adsorption capacity of urea. The experimental results showed that DL-Ti3C2Tx (LiOH-Ti3C2Tx) after treated by LiOH had the highest urea adsorption efficiency, and the urea removal rate of LiOH-Ti3C2Tx was still higher than 92% when the urea concentration was 500 mg/L. In the Simulated dialysate, Ti3C2Tx treated with three kinds of alkali solutions still maintained a good adsorption efficiency for urea, and still had a certain adsorption capacity after recycling for four times. Biocompatibility experiments showed that Ti3C2Tx in different concentrations did not cause hemolysis of erythrocyte, and had no obvious damage to vascular endothelial cells. This study greatly improves the urea adsorption efficiency of MXene, which has a broad application prospect in the selection of adsorbent for wearable artificial kidney.

Hydrogen production from urea in human urine using segregated systems

Removal of nitrogen compounds through biological processes represents the highest energy consumption in conventional centralised wastewater treatment facilities. Alternatively, segregated systems, where wastewater is treated at its source, present the potential to provide value to nitrogen-rich compounds contained in wastewater like urea. This paper demonstrates the feasibility of a novel process to recover energy from human urine based on the pre-isolation of urea to decrease the energy requirements for its thermal decomposition compared to the conventional thermal treatment when in solution, followed by its decomposition into hydrogen. Herein, urea is separated from an aqueous solution by adsorption onto activated carbon. Thermal urea desorption and decomposition into ammonia and CO₂ at 250 °C leads to full regeneration of the carbon, showing a constant adsorption capacity for at least 5 consecutive adsorption/desorption cycles. Finally, when the regeneration and urea decomposition step is coupled to an ammonia decomposition catalyst, hydrogen is produced to be used as an energy fuel. This process opens the door to a new way of circular economy by energy recovery from hydrogen-rich components in segregated wastewater streams. Preliminary energy balances show that the adoption of this energy recovery system in a city of 160,000 inhabitants would lead to a daily hydrogen production of 430 kg, with a net energy production of 2,500 kWh/day. In addition, such waste-to-energy process would lead to energy savings of 4,600 kWh/day in a conventional wastewater treatment plant reducing its energy consumption by around 35%.

Evolution of low molecular weight organic compounds during ultrapure water production process: A pilot-scale study

This study evaluated the evolution of low molecular weight organic compounds in ultrapure water (UPW) production using a pilot-scale UPW production system and an ultrafiltration-reverse osmosis (UF-RO) system. During UPW production, a dissolved organic carbon (DOC) removal efficiency of 99.4% was achieved with a feedwater DOC level of 1.42 mg/L. The pretreatment, make-up, and polishing stages accounted for 85.3%, 13.7%, and 0.4% of DOC removal, respectively. Urea, trichloromethane, and dibromochloromethane persisted throughout UPW production process, contributing 24.7%, 9.2%, and 22.6%, respectively, to the final effluent DOC level of 8.1 μg/L. The pretreatment and make-up stages of the UPW production process could remove N-nitrosodimethylamine, chloral hydrate, dichloroacetonitrile, and tribromomethane. The UF-RO system could remove approximately 90% of DOC. However, the proportion of halogenated disinfection by-products (DBPs) in the DOC increased by 1.4-4.5 times in the RO effluents. RO could completely reject haloacetaldehydes. However, RO could not completely remove trichloromethane, tribromomethane, bromodichloromethane, and dibromoacetonitrile, which remained the main halogenated DBPs in the RO effluents.

Adsorption of Phenol onto Aluminum Oxide Nanoparticles: Performance Evaluation, Mechanism Exploration, and Principal Component Analysis (PCA) of Thermodynamics

The removal of phenolic compounds from aqueous solutions using novel adsorption techniques becomes a key research item. Of those, nanoparticles in particular, the low-cost and the high-strength aluminum oxide nanoparticles showed promising results in pollutant uptake and increase in the adsorption efficiency. This study examined various physicochemical process parameters such as temperature, pH, initial phenol concentration, and adsorbent doses, in addition to the impact of those parameters on the adsorption removal mechanism of phenol. The results highlighted that aluminum oxide nanoparticles successfully exhibited superior phenol removal from an aqueous solution in addition to a high potential regeneration of the consumed nanoparticles by HCl. For the adsorbent mass of 0.5 g, phenol adsorption uptake reached 92%. Kinetic studies performed using several models demonstrated the data best fitting with a pseudo-second-order kinetic model. Examining equilibrium studies of various isotherms, the adsorption data of phenol into aluminum oxide nanoparticles was confirmed to be controlled by film diffusion and best represented by the Langmuir isotherm. The maximum capacity of adsorption was 16.97 mg/g. For thermodynamics studies, the results indicated that the adsorption process can vary between endothermic and exothermic reactions. Such relative differences in heat generation and spontaneity in adsorption processes were demonstrated and confirmed by principal component analysis (PCA). This evidence is key for future investigations for the efficiency of adsorption conditions concerning the contaminant type and adsorbate compounds.

Urea removal in reclaimed water used for ultrapure water production by spent coffee biochar/granular activated carbon activating peroxymonosulfate and peroxydisulfate

This study evaluated the performance of spent coffee biochar (SCBC)/granular activated carbon (GAC) activating peroxymonosulfate (PMS) and peroxydisulfate (PDS) for urea degradation in reclaimed water used for ultrapure water production. Results showed that catalyst and oxidant wielded a great influence on urea removal. Of them, the GAC-PMS system could completely remove urea at the least oxidant (1 g/L) and catalyst dosage (0.2 g/L). GAC activating PMS mainly depended on graphite C structure and minor oxygen functional groups. However, the amounts of urea removed by 600BC-PMS and 900BC-PMS were 57% and 70%, respectively. In the PDS system, the urea removal through GAC-PDS could reach 90%, which mainly depends on the graphite C structure of GAC. Using the same conditions, the urea removal of 900BC-PDS was similar to GAC-PDS, so it has some potential as an alternative to commercial GAC.

Adsorption of uremic toxins using biochar for dialysate regeneration

Numerous studies have shown that patients with COVID-19 have a high incidence of renal dysfunction. However, the dialysis supplies, including dialysates, are also severely inadequate in hospitals at the pandemic centers. Therefore, there is an urgent need to develop materials that can efficiently and rapidly remove toxins and thus regenerate dialysate to make this vital resource remains readily available. In this work, by simple carbonization and activation treatment, the porous activated carbon from waste rubber seed shell (RAC) was prepared. The adsorption results showed that the maximum adsorption capacities of the obtained RAC for creatinine and uric acid were 430 mg/g and 504 mg/g, respectively. Significantly, the adsorption process can be close to the equilibrium state within 0.5 h, which proved the ultra-fast adsorption response capacity of RAC. Further, the thermodynamics analysis results showed that both the creatinine and uric acid adsorption processes were monolayer, exothermic, and spontaneous. The adsorption kinetics results indicated that the adsorption process of the two uremic toxins followed the pseudo-second-order rate model and was dominated by chemisorption. The instrument analysis results reflected the efficient adsorption of the RAC for the above uremic toxins which might be due to the dipole-dipole interaction between the dipolar oxygen-containing groups of the surface of RAC and the dipoles of the toxins. Moreover, the formed hydrogen bonds between the oxygen groups and the toxins also played an important role. In all, the as-prepared RAC has the potential to efficiently remove major toxins from the dialysate and can be used in in vitro dialysis of numerous patients during the current COVID-19 pandemic.

A critical review on challenges and trend of ultrapure water production process

The recent and vigorous developments in semiconductor technology strictly request better quality and large quantity of ultrapure water (UPW) for their production. It is crucial to secure a large amount of raw water for the future development of UPW production. Using reclaimed water as alternative raw water source to produce UPW is therefore considered the feasible trend and solution for sustainable use of water resources towards a common future practice in UPW production. The challenge of using reclaimed water is due to its higher content of organic pollutants, especially small molecule organic pollutants such as urea, which are difficult to remove through traditional UPW production process. Consequently, improving the existing UPW production process to meet the water standard desired in the semiconductor industry is essential. This paper reviewed the current traditional processes for removing organic matters in UPW production, including ion-exchange (IX) adsorption, granular activated carbon (GAC) adsorption, reverse osmosis (RO) and ultraviolet (UV) irradiation. The potential problems in the actual UPW production process were identified when using reclaimed water as raw water source. A new strategy of applying the advanced oxidation process (AOPs) to UPW production as a supplementary unit to guarantee UPW quality was proposed. Its feasibility and research focus were then analyzed and discussed in obtaining a new solution for a future development of the UPW production process.